Diversity receiving system



Jan. 14, 1947.

mxvmnsw? RECEIVINGHSYSTEM w. LYONS 12,414,111

m ned March.22, 1944 INVENI'OR WALTER ONS Z fur-0v ATTORNEY Patented Jan. 14, 1947 DIVERSITY RECEIVING SYSTEM Walter Lyons, Wenonah, N. J assignor to Radio Corporation of America, a corporation of Delaware Application March 22, 1944, Serial No. 527,572

11' Claims.

This invention relates to diversity receiving systems and more particularly to that type of radio receiving equipment which makes provision for combining the signal energy derived from di-fferent antennae and separately amplified in different radio receiving circuits.

It is common practice to provide a plurality of heterodyne receivers each connected to an individual receiving antenna, the several antennae being spaced apart so as to minimize the effects of fading. In some of the well-known diversity receiving systems, means are provided for automatically switching the signals through alternative amplifiers so as to maintain connection of a responsive device to the particular amplifier which at any moment is receptive of the strongest signals.

In another group of developments in diversity receiving systems the practice is adopted of feeding rectified output potentials from the intermediate frequency amplifiers to a common load which is permanently coupled to each of these intermediate frequency amplifiers operating in parallel and contributing to the output one at a time where the inputs are unequal by a substantial amount, or both supplying output where the inputs are substantially equal. My invention belongs in this group. I have found that reception can be greatly improved when the gain of each amplifier is automatically adjusted so that the individual amplifiers shall be capable of contributing substantially equally to the common load regardless of the working strength of the incoming signals applied to each amplifier. Thus the working signal may be 10 microvolts or 1,000 microvolts intensity and all amplifiers will be capable of contributing equally to the common load for equal signal input at the antennas. This improvement is particularly needed in cases where a common automatic gain control circuit is provided for the individual amplifiers and where the potentials utilized in the AVG circuit are derived from the average signal strength of all of the individual diversity receivers.

It is an object of my invention to provide means for automatically adjusting the gain in each of a plurality of receivers connected in a diversity system so as to compensate for variations in the relative input signallevels of the individual receivers and so that the output signal energy derived from each amplifier shall be of substantially equal magnitude.

Other objects and advantages of my invention will be made apparent in the detailed description to follow. This description is accompanied by a 2 drawing, the sole figure of which represents diagrammatically a preferred circuit arrangement for carrying out the invention.

Referring to the drawing, I show illustratively two receiving antennas l and 2 which are individually connected to radio frequency amplifiers 3 and 4. The output from each of these amplifiers is fed to an appropriate converter 5 and 6. A local oscillator 1 provides energy of a suitable frequency for heterodyning with the amplified signals in each of the converters 5 and 6, thus producing a heterodyne frequency which is then fed to the intermediate frequency amplifiers. One of these IF amplifiers includes the discharge tubes 1 l, 13, and 15. Another of these IF amplifiers includes the discharge tubes l2, l4, and 16. Each of the tubes H to l G inclusive is shown as a pentode, although other types of amplifier tubes may be employed, if desired.

The control grid in the first stage tube 11 of the one IF amplifier is connected to a common automatic gain control circuit I! through the secondary winding of a coupling transformer l8. This secondary winding is tuned by connecting an adjustable condenser 8 across its terminals. Likewise in the second IF amplifier the control grid of tube 12 in the first stage is connected to the common AVC circuit 11 through the secondary winding of a coupling transformer 20. This secondary winding is also tuned by means of an adjustable condenser 8.

In the second stage of each IF amplifier tubes 13 and 14 are shown with their control grids connected similarly to the input circuit connections of the first stage. That is to say, each of these second stage tubes is rendered subject to the common AVC potentials. The second stage input circuits are also fed with signal energy by virtue of inductive coupling to the output circuit in the first stage, as is conventional. The interstage couplings are shown in the one IF amplifier to include transformer 22 both primary and secondary windings of which are tuned to the IF frequency; and in the case of the second 1F amplifier, transformer 23 is used. In fact the said connections for the two IF amplifiers are identical in arrangement. A tuning condenser 8 is connected in shunt with the primary and with the secondary of each interstage coupling transformer.

The screen grids in the first two stages of both IF amplifiers are connected through series resistors 24 and 25 respectively to the positive terminal of the direct current operating source indicated as +B. Resistor 24 in series with an adjustable resistor 26 constitute a voltage divider whereby the normal voltage to be applied to the screen grids of tubes I I and I3 is obtained. Likewise resistor 25 in series with the adjustable resistor 21 constitute a similar voltage divider for obtaining the normal screen grid potentials to be applied in tubes i2 and i4. These screen grid potentials, however, are made automatically adjustable by means of two discharge tubes 28 and 29, the space paths of which are disposed in shunt with the adjustable resistors 26 and 21 respectively. Capacitors 9 are used to bypass to ground the alternating potentials which appear on any of the screen grids.

Each of the tubes 28 and Z9 is preferably of the triode type and their control grids are subjected to variable control biases, each in dependence upon the rectified signal output level of a different amplifier. This will be explained further after describing the circuit connections of the third stage in each of the IF amplifiers.

The control grid in tube l5, which is in the third stage of one IF amplifier, is coupled to the output circuit of tube l3 across transformer 33. Likewise the control grid in tube [6, which is in the third stage of the second IF amplifier, is coupled to the output circuit of tube M across transformer 3|. Each of the tubes I5 and I6 is provided with a cathode resistor 32. These cathode resistors are shunted by capacitors 33 as is usual. The screen grids in tubes l5 and it are connected to the +13 terminal through series resistors 34. The only gain control action in tubes l5 and I6 is, therefore, that which results from the self-biasing effects of the cathode resistors 32.

The output circuit of tube l5 includes a resonant circuit 35, the inductance of which constitutes the primary winding of an output transformer 36. Likewise, the output circuit of-tube i6 includes the resonant circuit 31, the inductance of which constitutes the primary winding in output transformer 38. The secondary windings of transformers 36 and 38 are both connected to a common load 39. The terminal of the load impedance 39 remote from the transformer windings is grounded. This load impedance may, therefore, be used to supply output potentials for feeding the signals to any suitable utilization device. The output terminals are indicated at 48.

In order that each of the IF amplifiers may have its gain somewhat slowly adjusted in accordance with the floating average of amplitudes of the signal output from the other amplifier, I have made the grid in tube 28 subject to the delayed influence of rectified output potentials from tube 16. correspondingly, the grid in tube 29 is made subject to the delayed influence of rectified output potentials derived from tube l5. Rectification in the one case is obtained in the space path within the twin diode tube 4|, which develops a D. C. potential drop, negative with respect to ground, in a resistor 520. This rectifier circuit is coupled to the output circuit of tube it; through capacitor 43. Rectification in the other case is obtained in the space path a within the twin diode tube 40. The D. C. potential so developed in resistor 52a results from coupling this rectifier circuit to the output circuit of tube l through capacitor 42. are each shunted by a capacitor 53 for draining off to ground any existing high frequency components.

The grids in tube 28 and 29 are connected respectively to the detector circuits of the diode Resistors 52a and 52c.

4 space paths 0 and a through resistors M of relatively high impedance. Resistors 44 in combination with capacitors 55 provide time constant circuits which are relatively long. They may be adjusted, for example, to produce a delay action of as much as 10 seconds or more in the control of the tubes 28 and 29. The reason for introducing a considerable delay period into the compensating adjustment of the screen grid potentials as between the two receivers is that this action must be maintained distinct from that of the conventional AVC circuit, especially where the control potential of the latter are to be applied in common to both receivers in dependence upon the rectified output from the receiver delivering the strongest signals.

For the relatively slow adjustment of the gain in each receiver in dependence upon the fioating average of signal output intensity from the other receiver, assume first that antenna 2 is delivering signals of dominating intensity. Then the impedance of the space path in tube 23 is eventually increased in proportion to the intensity of the rectified signal potentials derived from tube It and rectified in space path c of the twin diode tube 3 l The effect of this increase of space path impedance in tube 28 is to increase the screen grid potentials, and hence the gain, in tubes H and I3. Assume now that antenna i delivers the stronger signals. Then the gain in amplifier tubes l2 and I4 i increased by raising the screen grid potentials of these tubes in'response to a resulting increase in the space path impedance of tube 29, where the signal output potential is derived from tube 15 and rectified in section a of the detector tube 40.

Rectification of the IF amplifier output for application to a utilization device, or audio frequency responsive unit, is accomplished by means of the space paths b and d in the diode tubes 40 and ll respectively. These space paths are in circuit with the common load 39. Path 11 is in series with the secondary of transformer 36. Path d is in series with the secondary of transformer 38. The two secondaries are tuned by parallel capacitances 8.

The common load 39 is preferably shunted by a capacitor 4'! for peak detection and filtering. Capacitor 49 serves to isolate the utilization device from D. C. components in the load 39. Its use is optional. The output leads are indicated at 48.

The automatic gain control circuit I? is connected through a resistor 50 to the common load 39, as is conventional. A capacitor 5i operates in conjunction with resistor 50 for obtaining the desired time constant in the operation of the AVG circuit. Resistors l9 carry the AVG potentials to the grids in the RF amplifiers 3 and 4, and in the tube stages H and i2. Bypass condensers iii are also used conventionally in association with resistors I!) for draining ofi to ground any A. C. potentials which may appear in the AVG circuit.

It Will be apparent that the retarded compensation of gain in the two receivers may be accomplished independently of the conventional AVC control if difierent stages of amplification are rendered subject to one and to the other of the two types of gain control. In this suggested modification it is unnecessary to apply the compensated gain regulation through the screen grids. as has been described in the foregoing illustrative embodiment. Instead, the tubes 28 and 29 may be used to apply a more positive (or less nega-' tive) bias to the control grid in one or more stages of one receiver in response to an increase in the output intensity of the other receiver.

My invention is, of course, capable of modification in various other ways, as will be understood by those skilled in the art, in order to meet diiferent requirements. The circuit shown and described is merely illustrative, although I have found it to be practical. In fact it appears to possess many advantages over diversity receiving systems heretofore known.

I claim:

1. In a diversity receiving system having two receivers each individually fed with signal energy which is collected by a separate antenna, each receiver being characterized by the inclusion of a plurality of amplifier stages having screen grid tubes, apparatus for equalizing the signal intensities delivered as rectified output from the respective receivers and applied to a common utilization device, said apparatus comprising a direct current source of screen grid potential shunted by potentiometers. which are individual to each receiver, two electronic circuit impedances each in shunt with a respective section of said potentiometers, said sections beingconnected to the negative terminal of said source, means for rectifying an output component from each receiver, and means for utilizing each said output component from a respective receiver to vary the said electronic circuit impedance appropriate to the other receiver.

2. In a diversity receiving system having two receivers separately fed with signal energy collected by antennae which are spaced apart, each receiver being characterized by the inclusion of a plurality of amplifier stages having screen grid tubes, apparatus which compensates for inequalities of receptive conditions at the two antennae comprising means for separately rectifying the output from each receiver, electronically controlled potentiometric means for varying the screen grid potentials in, and hence the sensitivity of, each receiver in dependence upon the amplitude of the rectified output from the other receiver, means for rectifying a component of said output from the receiver which delivers the strongest signals and for utilizing the same as a common gain control potential, and means for applying said common gain control potential to certain amplifier stages in both receivers.

3. A diversity receiving system comprising two receivers each fed with signal energy by its own antenna, at least one amplifier stage in each receiver having a screen grid tube arranged for regulating its amplification ratio by control of the screen grid potential applied thereto, a direct current source for energizing the discharge tubes in said amplifier stages, two voltage dividers connected across the terminals of said source, connections from an intermediate point on one of said voltage dividers to the screen grid of said screen grid tube of at least one amplifier stage in one of said receivers, similar connections from a corresponding point on the other voltage divider to the screen grid of said screen grid tube of at least one amplifier stage in the other receiver, and two discharge devices having their space paths connected each in shunt with a portion .Of one of said voltage dividers respectively, each of said discharge devices having a grid for controlling the impedance of its space path, and the grid of the discharge device which serves to regulate the screen grid potentials in one receiver being controlled by output potentials derived from the other receiver, respectively.

' 4'. In a diversity receivingsystem having a plurality of receiversseparately fed with signal energy collected by antennae which are spaced apart, a plurality of amplifier stages in each of said receivers, a multigrid discharge tube in each stage, a direct current operating potential source connected to the tube electrodes, a voltage di vider individual to each receiver and connected across terminals of said source, a, controllable discharge device in shunt with a portion of each said voltage divider, a gain control circuit for each receiver, comprising connections from a point on its respective voltage divider to at least one of the'screen grids in its amplifier tubes, and means recipro'callyoperable as between the gain control circuits of the diiferent receivers whereby an amplitude increase in the output of rectified potentials delivered by one receiver serves to control the discharge device which is in shunt with the voltage divider portion appropriate to a different receiver, wherein the gain is thus increased.

5. In a diversity receiving system having two receivers and a separate antenna feeding signal energy to each receiver, a load common to certain output components derived from the two receivers, means for separately rectifying other output; components from each receiver, a multigrid discharge tube in each amplifier stage of said receivers, a direct current source for activating said tubes, two voltage dividers connected across the terminals of said source, each of said dividers having a tap thereon for feeding screen grid potentials to certain of said multigrid tubes in the respective receivers, and electronic means operably associated with said voltage dividers and subject to control by said rectifying means in such manner as to cause said screen grid potentials as applied to the screen grids in one receiver to be varied as a direct function of the amplitude of the rectified signal component separately derived from the other receiver.

6. The combination according to claim 5 and including an automatic gain control circuit common to both receivers, said circuit being responsive to potential variations in said common load.

7. In a diversity receiving system comprising two receivers A and B, each receiver having its input circuit coupled to a separate antenna and having a multistage amplifier, a multigrid discharge tube in each amplifier stage, a direct current source havin suitable connections to the electrodes of said tubes for activating the same, means for rectifyin and combining in a common loa impedance certain output component-s derived from the two receivers, further means for rectifying and segregating other output components from each individual receiver, a pair of triode discharge tubes each separately controlled :by one and the other, respectively, of said segregated output components, and means individual to each receiver for applying variable screen grid potentials to certain of its amplifier discharge tubes, the last said means being effective through said triode discharge tubes to control the gain in receiver A in dependence upon the amplitude of rectified signal output derived from receiver B, and vice versa.

8. A diversity receiving system in accordance with claim 7 and including an automatic gain control circuit operative in dependence upon the amplitude of the output components which are combined in said common load, said gain control circuit being connected to the control grids in certain of the amplifier stages of both receivers.

9. A diversity receiving system in accordance with claim 7 and including in each of the stated means for applying variable screen grid potentials a pair of voltage dividers each individual to the screen grid potential supply circuits of the separate receivers, a portion of each voltage divider being shunted by the space path of a respective one of said triode discharge tubes.

10. A diversity receiving system comprising two receivers each fed with signal energy by its own antenna, a plurality of amplifier stages in each receiver, certain of said amplifier stages having input circuits which include a common control grid biasing source, rectifier means individually coupled to the output circuit of the final stage in each receiver for detecting the audio frequency components of the received signals, a common load for said components, a gain control circuit responsive to potential variations across said common load for influencing the effective value of said grid biasing source, other rectifier means individually coupled to the output circuit of the final stage in each receiver, a pair of discharge tubes each having an input circuit subject to control by rectified output from said other rectifier means associated with a respective receiver, and having an output circuit eifective to influence the gain in a different receiver, and two time constant circuits each having a delay period of the order of ten seconds or more in the singular control of each of said pair of discharge tubes, whereby a retarded equalization of the output from the two receivers is obtained.

11. In a diversity receiving system comprising two receivers each fed with signal energy by its own antenna, each receiver having output connections to a common load and further output connections to each of two gain control systems one of which is effective for adjusting the gain in both receivers in dependence upon the predominating output from either receiver, and the other gain control system constituting means for causing the gain in each receiver to be varied as a direct function of the output from the other receiver, a circuit organization for operating said gain control systems which comprises means for rectifying an output component from each receiver, means for utilizing said rectified output for purposes of common gain control in both receivers, means for separately rectifying another output component from each receiver to be applied as a gain control potential in the other receiver, and means for introducing considerably more delay in the application of the last said gain control potentials than in the common gain control potentials.

WALTER LYONS. 

